With increasing demands on commercial data transmission applications such as radio frequency identification (RFID) tag applications, attention has been applied to the design of compact integrated and directional antennas with circular polarization and good matching performance. Traditionally, engineers prefer deploying patch antennas in RFID systems because patch antennas have many advantages such as having a low profile, being conformal to planar surfaces, and the ability to integrate the antenna with a printed circuit such as a monolithic microwave integrated circuit (MMIC).
Antennas for RFID readers should be directional, but conventional circularly polarized patch antennas suffer from narrow bandwidth and the directivity of patch antennas is not high enough for them to function as a good RFID base station antenna. Some proposed techniques such as increasing the thickness of the patch antenna, employing a capacitive coupled feed or an L-probe feed can overcome the narrow bandwidth problem. Furthermore, a patch antenna array is one way to achieve high directivity signal radiation, but this comes at the cost of a large overall size and cost of the patch antenna array.
For cost effectiveness and space utilization, wideband, high gain, low profile and circularly polarized wave radiating antennas that can accommodate several communication systems are in high demand. In particular, antennas with directional radiation patterns are of interest as they can be mounted on walls, or other objects such as vehicles, without degrading their electrical properties. Axial mode helix antenna designs are another suitable candidate to be used as a RFID base station antenna. Helix designs produce a directional antenna pattern, generate circularly polarized radio waves, and have a wide operational frequency bandwidth. However, the large pitch angle for the traditional axial mode helix antenna prevents the fabrication of a low-profile antenna. The circumference of the axial mode helix is around one wavelength and the optimum pitch angle according to Kraus is 12.5° [see: Kraus, J. D., “Antennas”, New York: McGraw-Hill, chapter 8, pp. 333-338]. To achieve a relatively narrow beamwidth helix at 915 MHz, the number of windings of the helix should at least 10. In other words, the physical height of the axial mode helix will be too high to be a good RFID base station antenna.
It is known that if a conventional circular helix is deformed into an elliptical one, then circular polarized waves can be restored by winding two helical antennas on a common elliptical core [see: Wu, Z. H.; Che, W. Q.; Fu, B.; Lau, P. Y.; Yung, E. K. N.; “Axial mode elliptical helical antenna with parasitic wire for CP bandwidth enhancement” Microwaves, Antennas & Propagation, IET, Volume 1, Issue 4, August 2007 Page(s):943-948].